A child was bitten by an emotional support dog on a Southwest Airlines plane in Phoenix late Wednesday, adding to the debate about the risks of an increasing number of in-flight animals.

The teeth of the dog scraped the child's forehead as the passenger approached it after boarding a Portland-bound flight, according to Southwest.

The child, whom the airline says was 6 or 7 years old, was treated by paramedics for a minor injury, the airline says, and continued on the flight. Southwest did not disclose where the passenger lives.

The unidentified dog owner remained in Phoenix when the flight departed about 20 minutes late, the airline said.

"As always, the safety of our customers is our highest priority,'' Southwest said in a statement.

At least one passenger was bothered by the incident, posting about it on Twitter.

In a later tweet, he said, "When dogs are biting 6 year old kids on planes, it may be time to reconsider rewriting your 'safety' policy and procedure manual.''

Some Twitter users replied that the child should have not approached the medium-sized dog.

"When kids are touching service dogs, it may be time for parents to wise up,'' one post said.

The incident comes as at least two major airlines are tightening their rules on emotional support animals, which are different than trained service animals. There is growing concern about fake online authorizations for emotional support animals, which are exempt from airlines' usual pet fees. Southwest, for example, charges $95 one way to bring a small animal that fits underneath the seat in a carrier.